I think it's just for the toilet. The sink should be close enough to the galley gray tank to drain into it. I always have both black tank valves pulled out when I use the flush so I'm not sure on that one. Also there should be a sticker on the side of the trailer between the slides marking the black tank valve for the back bath.
2011 F350 Dually 13,300 GVWR 3.73 ratio
2013 WildCat 344QB with outside kitchen